Oscar Pistorius is due to hear more witness testimony from the night he shot dead Reeva Steenkamp - after breaking down in court as a neighbour described the moments he tried to save his mortally wounded girlfriend.

The athlete will return to the Pretoria courtroom where prosecution witness Johan Stipp yesterday said he went to Pistorius' home after hearing shots fired on the night Ms Steenkamp was killed.

The radiologist said: "I got inside and there was a lady lying on her back.

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"I also noticed a man kneeling on her side, on the left side. I remember the first thing he said when I got there was 'I shot her, I thought she was an intruder. I shot her.'"

Video:Pistorius Trial: Behind The Scenes

He said he did not recognise Pistorius, and described how he tried in vain to resuscitate Ms Steenkamp.

"She had no pulse in her neck, she had no peripheral pulse, she had no breathing movements that she made," he said.

"She was clenching down on Oscar's fingers as he was trying to open her airway.

"I tried to do a jaw lift manoeuvre, to try to open the airway, but it was very difficult with the clenching down.

Video:Pistorius Breaks Down In Court

"All during that time, there wasn't any signs of life that I could see.

"I opened her right eyelid. The pupil was fixed dilated, and the cornea was milky - in other words, it was already drying out.

"So to me it was obvious she was mortally wounded.

"I looked at the rest of her body and I noted she had a wound in her right thigh, also a wound in the right upper arm.

"During that time … Oscar was crying all the time. He prayed to God to 'please let her live, she must not die'.

"He said at one stage while he was praying 'he will dedicate his life and her life to God if she would just only live and not die that night'."

He added that Pistorius looked "sincere": "He was crying, there were tears on his face. He was actively trying to assist her."

Sky's Alex Crawford, who was in court, said Pistorius was openly sobbing as the evidence was given.

Earlier, Pistorius' defence team said it would have been "impossible" for neighbours to hear screams on the night Ms Steenkamp died.

Lawyer Barry Roux said neighbour Charl Peter Johnson's claim that he heard gunshots followed by a woman screaming was incorrect.

"At the time you heard the deceased, she was in a locked bathroom," said Mr Roux.

"You cannot hear it inside your house ... Even standing on the balcony, it would have been impossible to hear the screams."

Mr Johnson was giving evidence for a third day.

Both Mr Johnson and his wife Michelle Burger insist they heard a woman's screams during the shooting.

The issue of the sound of a woman screaming is a key point for the prosecution. The claims by neighbours that they heard the screams suggest Pistorius would have known it was Ms Steenkamp, rather than an intruder, as he fired.

Mr Johnson also claims he heard five or six shots on the night of the killing.

Video:Lawyer Undermines Key Witness

But Mr Roux pointed out that in his initial notes he wrote that he "did not count the number of shots fired".

Mr Roux said: "Three hours later you changed this paragraph, and you corrected little things.

"That is a sign that you revisited that note and amended it."

He also suggested Mr Johnson had given evidence to fit with his wife's account of the evening.

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Gallery: Oscar Pistorius Trial: Key Figures

June Steenkamp, mother of Reeva Steenkamp.

Mr Johnson said he and his wife had spoken after the incident about what had happened, but that he did not hear her give evidence or speak to her about what she said in court.

Mr Roux said: "Your interpretation today is a designed one, it's to incriminate."

On top of the premeditated murder allegation, Pistorius faces a charge of illegally possessing ammunition and two further counts related to shooting a gun in public in two separate incidents before the killing.

He denies all the charges against him and maintains he shot Ms Steenkamp after mistaking her for an intruder.

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Gallery: Pistorius: A Sporting Pioneer

Pistorius was born in Johannesburg on November 22, 1986. A congenital condition meant he had no bones in his lower legs.

The athlete could face 25 years in jail if he is found guilty by Judge Thokozile Masipa. South Africa does not have trials by jury.